Many types of photodetector structures have been proposed for diverse applications including optical communication systems. For many of these applications, it is desirable that the photodetector structure exhibit gain without the need for external, electrical amplifiers. Two well known types of photodetector structures that exhibit gain are photoconductors and avalanche photodetectors.
In addition to exhibiting gain, an avalanche photodetector should have relatively low noise associated with the avalanche multiplication process, i.e., the avalanche process should not reduce photodetector sensitivity by increasing noise. In silicon avalanche photodetectors, low noise avalanche multiplication is relatively easy to achieve as the ratio of the ioization coefficients for electrons and holes useful at wavelengths longer than 1.0 .mu.m and other materials must be used at such wavelengths. However, in avalanche photodetectors using Group III-V compound semiconductors and structures analogous to those used for silicon, the ratio of the ionization coefficients for holes and electrons is typically close to unity, and the noise associated with the avalanche multiplication process is large. That is, the ratio of the ionization coefficients for bulk Group III-V compound semiconductors is typically close to unity.
Various structures have been disclosed in attempts to reduce the noise associated with the avalanche process in avalanche photodetectors using compound semiconductors. One such photodetector is described in U.S. Pat. No. 4,476,477 issued on Oct. 9, 1984 to Capasso, Tsang and Williams. This photodetector, commonly termed a staircase by those skilled in the art, uses the energy imparted to carriers as they cross energy band discontinuities to impact ionize and produce avalanche multiplication. As the discontinuity occurs primarily in one energy band, only one type of carrier impact ionizes and, consequently, the noise associated with the avalanche process is relatively low. The structures described used a graded, i.e., varying, bandgap to obtain the required energy band profile.
Other types of photodetector structures have been proposed including several using superlattices. A superlattice is a structure having a periodicity which differs from that of the underlying crystal lattice. That is, a superlattice may be viewed as comprising interleaved layers of compositions A and B. If one layer, e.g., that with composition A, is sufficiently small, quantum effects are significant and the device may be called a quantum well structure. An exemplary photodetector using such a structure is described in The Journal of Vacuum Science and Technology, B1, pp. 376-378, April-June, 1983. The structure described had interleaved layers of GaAs and GaAlAs which were uniformly doped n-type. The GaAs quantum wells and barrier layers were between n-type Ga.sub.0.6 Al.sub.0.4 As cladding layers. The device was operated at a sufficiently low temperature so that essentially all the electrons were trapped in the quantum wells. Application of a bias voltage to an unilluminated structure did not result in significant current flow as there were few carriers present outside the quantum wells. The bias was applied perpendicular to the quantum wells, i.e., to the two cladding layers. However, when the device was illuminated and photons absorbed in the quantum wells, electrons were excited from the wells and current flowed in the external circuit, if the photons had energy sufficient to excite electrons from the wells. This device may thus be thought of as a particular type of photoconductor.
The analysis given by the authors is interesting because they found substantial photoconductive gain, approximately 10,000, but also an exceedingly slow device response time of approximately one second which was determined by the electron capture time. There were other effects, including a possible avalanche multiplication effect, mentioned, the significance, and indeed the presense, of which the authors were not certain. For the particular effect just mentioned, it was hyposthesized that electrons traveling perpendicular to the quantum wells might scatter electrons from within the wells. However, the effects of such an avalanche process were neither described nor observed by the authors.